Being simply in the presence of one who is amused, or happy, or sad, causes us to feel amused, or happy, or sad ourselves--or, at least, has that tendency--even if we do not know from what cause the emotion which is communicated to us proceeds.

A person of a joyous and happy disposition often brightens up at once any little circle into which he enters, while a morose and melancholy man carries gloom with him wherever he goes.
Eloquence, which, if we were to hear it addressed to us personally and individually, in private conversation, would move us very little, will excite us to a pitch of the highest enthusiasm if we hear it in the midst of a vast audience; even though the words, and the gestures, and the inflections of the voice, and the force with which it reaches our ears, were to be precisely the same in the two cases.

And so a joke, which would produce only a quiet smile if we read it by ourselves at the fireside alone, will evoke convulsions of laughter when heard in a crowded theatre, where the hilarity is shared by thousands.
A new element, indeed, seems to come into action in these last two cases; for the mental condition of one mind is not only communicated to another, but it appears to be increased and intensified by the communication.

Each does not feel _merely_ the enthusiasm or the mirth which would naturally be felt by the other, but the general emotion is vastly heightened by its being so largely shared.

It is like the case of the live coal, which does not merely set the dead coal on fire by being placed in contact with it, but the two together, when together, burn far more brightly than when apart.
_Wonderful Power of Sympathy_.
So much for the reality of this principle; and it is almost impossible to exaggerate the extent and the magnitude of the influence it exerts in forming the character and shaping the ideas and opinions of men, and in regulating all their ordinary habits of thought and feeling.